1. Field of Invention
The present invention relates to a process for the simultaneous removal of BOD and phosphate from a liquid containing ammonium, BOD and phosphate and to an apparatus for use in such a process.
2. Description of Related Art
In municipal and industrial water applications phosphate-polluted wastewater is generated and phosphate removal techniques may be applied for the wastewater treatment. Conventional processes are based on the fixation in activated sludge or on the precipitation of phosphate as calcium or iron salt. The latter process generates huge amounts of a water-rich sludge, which has to be disposed of at continuously increasing costs. To minimise disposal costs, the sludge is often mechanically dewatered prior to disposal. Typically, even after dewatering, the water content of the sludge still amounts to 60% to 85% and a relatively large part of the disposal costs comes from the expensive disposal of water. Due to the high water content and the low quality of the waste sludge, reuse of phosphate is not an economically attractive option.
U.S. Pat. No. 5,126,049 describes a process for the treatment of raw water, comprising the biological denitrification of water in the presence of bacteria to produce a supply of water having a reduced nitrogen content and bacteria containing cell-bound nitrogen. The supply of water having reduced nitrogen content is aerated to release said cell-bound nitrogen in the form of ammonium ions (not nitrate or nitrite ions). The ammonium ions are precipitated in the form of struvite by treating the supply of water with magnesium ions and phosphate ions in a reactor to produce a substantially nitrogen free supply of water. In order to attain optimal conditions for the biological reaction, alcohol, sugar or organic acid is added as a carbon source for the bacteria. The precipitation preferably includes treating the water with magnesium and phosphate ions to precipitate the ammonium ions in the form of struvite (MgNH4PO4, MAP). This purification process requires at least four reactors and vessels: a denitrification reactor, an aeration vessel, a second reactor for the formation of struvite and a filtration or sedimentation vessel (tank).
EP1496019 (WO0308699) discloses a process and an apparatus for recovering magnesium ammonium phosphate as MAP crystals from wastewater containing high concentration organic substance, phosphorus and nitrogen such as a digested supernatant liquor of human waste sewage and septic tank sludge, a digested liquor of sludge and chemical plant wastewater. The sludge treatment process comprises treating a sludge mixed liquor formed in the biological treatment system for organic wastewater in an anaerobic digestion tank to effect the digestion of sludge, simultaneously adding a magnesium source to the anaerobic digestion tank to allow crystals particles of MAP to form and grow in the anaerobic digestion tank, withdrawing a sludge mixed liquor containing the crystals of MAP from the anaerobic digestion tank, separating and recovering solids containing MAP crystal particles from the withdrawn sludge mixed liquor, and returning part of the sludge mixed liquor after separation and recovery of MAP crystal particles to the anaerobic digestion tank.
Alternative processes may comprise the crystallisation of struvite, e.g. by using a suitable seed material like sand or minerals (Crystalactor® process) or by using granules (“Phosnix” process). In a process of the art, using a Crystalactor®, waste water is treated in an anaerobic biological reactor because of the low sludge production, the low energy consumption and the biogas production. The effluent is polished in an aerobic biological treatment plant. Phosphate is removed by struvite crystallization in a second reactor, by feeding MgCl2 and NaOH solutions into a part of the effluent of the anaerobic stage. In the “Phosnix” process, another process of the art, waste water containing phosphate is fed into a reaction vessel where magnesium is dosed, the pH is controlled and fine MAP crystals are formed. In the bottom part of the reaction vessel, MAP granules fluidized by air are retained and fine MAP crystals stick on the surface of the granules growing their size. Grown up MAP granules are discharged periodically from bottom of the reaction vessel.
A disadvantage of the process and the apparatus of the prior art is that it only provides facilities to remove phosphate, but BOD (biological oxygen demand) is not removed or BOD and phosphate have to be removed in different reactors. Another disadvantage is that extra chemicals may be needed to control the pH.